This invention generally relates to metal components. More specifically, the invention is directed to methods and devices for evaluating the thermal exposure of metal components which have been exposed to a variety of service conditions.
A variety of metal alloys are frequently used in industrial environments which include extreme operating conditions. For example, the alloys may be exposed to high temperatures (e.g., 500° C. or higher) for a long duration. Moreover, the alloys may be subjected to repeated temperature cycling, e.g., exposure to high temperatures, followed by cooling to room temperature, and then followed by rapid re-heating. As an example, gas turbine engines are often subjected to repeated thermal cycling during operation. Furthermore, the standard operating temperature of turbine engines continues to be increased, to achieve improved fuel efficiency. After gas turbine engine components are exposed to these conditions, they often undergo significant metallurgical changes, e.g., in chemistry or microstructure.
Monitoring the temperature of the turbine components can be extremely important for predicting engine failure, and for designing and optimizing the turbine engines. One technique for monitoring the temperature is sometimes referred to as “mapping”, in which the average exposure temperature of the component (e.g., a blade) is recorded—during actual service, or by using test coupons under experimental conditions. Mapping of the temperature profile of a turbine component is very useful for evaluating turbine design; estimating metallurgical changes, and providing estimates as to the remaining life of the turbine component.
In general, many different techniques are available for estimating the temperature history of a component, e.g., a blade or other rotating member within a turbine engine. For example, the component can be physically sectioned after service, and evaluated with a number of devices and techniques, such as optical microscopy, scanning electron microscopy (SEM), X-ray diffraction, and transmission electron microscopy (TEM). The temperature profile can then be estimated from this evaluation. Documented time/temperature/characteristic feature relationships, similar to the well-known Larsen-Miller parameters developed for the testing of metal alloys, are useful for this purpose.
Non-destructive techniques for temperature evaluation have also been described in the art. For example, a method for evaluating the remaining life of a high-temperature turbine component is described by H. Takehisa et al, in JP-2001124763A2. According to this technique, parts formed from rhenium-containing nickel alloys are examined to determine particle size or the precipitation amount of a gamma prime phase for the alloy, after high-temperature exposure. It appears that the remaining life of the component is estimated by determining the time at which the degraded phase for a pre-determined precipitation amount is detected.
In U.S. Pat. No. 4,923,308 (Watanabe et al), a process is described, for determining the relative temperature distribution at a surface of a high-temperature component used in an oxygen-containing environment. In this process, a wheel or other test specimen is formed initially. The wheel is made of silicon carbide or silicon nitride, and is exposed to a temperature environment which simulates that of the component being evaluated. The wheel is then cut in the shape of a vane, and oxygen concentration is measured for the wheel surface. The oxygen concentration distribution is said to correlate to the surface temperature distribution, thereby providing an estimate as to the component's temperature history.
U.S. Pat. No. 4,970,670 (Twerdochlib) describes a system for monitoring the temperature of a plurality of turbine blade shroud segments. Each shroud contains some feature (“indicia”) which is monitored by a sensor. In some embodiments, the sensor appears to sense eddy currents generated in each segment during movement of the shroud segment. A temperature sensor is responsive to changes in the temperature of the eddy current sensor. Input signals produced by the eddy currents are subjected to a correction mechanism, which in turn appears to provide data regarding the temperature history of the shroud segment.
Various conventional devices are also frequently used for the temperature-evaluation of metal components. Commercial examples are thermocouples, optical pyrometers, and black-body temperature sensors. Details and features for these types of instruments can be found in a wide variety of references.
The techniques discussed above for evaluating the temperature history of a component may be useful in a number of situations. However, most of those techniques have some notable disadvantages. For example, the technique in which a component is sectioned for examination is a destructive test which can be laborious and time-consuming. Furthermore, the conventional, temperature-measuring devices also have some drawbacks—especially when used in a harsh environment. For example, the accuracy of pyrometers can suffer because of “noise” caused by the passage of very hot particles through the component. Radiation pyrometers are also susceptible to interference from reflections and radiation originating from regions other than that which is under evaluation. Moreover, thermocouples, while being relatively inexpensive, are often too fragile for extended use in a harsh environment.
Some of the other non-destructive techniques mentioned above appear to require laborious procedures for temperature evaluation. Moreover, they may require a complex arrangement of sensors. The accuracy of some of these techniques may also be uncertain. For example, the process described above in U.S. Pat. No. 4,923,308 utilizes silicon-based wheels in place of the actual metal part being evaluated. Since the heat transfer characteristics of the silicon material may differ significantly from that of the metal component, the temperature measurement may not adequately correspond to the actual temperature exposure of the component.
It should thus be apparent that new techniques and systems for evaluating the thermal exposure of metal components would be of considerable interest in the art. The techniques should be non-destructive to the component being examined. They should also be accurate and relatively easy to carry out for a variety of metal substrates. Moreover, the techniques should be especially suitable for providing temperature profile measurements for superalloy components exposed to relatively high temperatures.